This invention relates to speed ratio control of a toroidal continuously variable transmission.
Tokkai Hei 5-39847 published by the Japanese Patent Office in 1993 discloses a toroidal continuously variable transmission. This toroidal continuously variable transmission comprises an input disk and output disk which can rotate relative to each other on a common axis, and a pair of friction rollers gripped between these disks. The toroidal continuously variable transmission varies an inclination (gyration angle) of the friction roller, and transmits the rotation between the disks at an arbitrary speed ratio. The gyration angle of the friction roller varies due to the displacement of trunnions supporting the friction roller in the direction perpendicular to the rotation axis due to an oil pressure. The displacement of the trunnion varies the position of the contact points with the input disk and output disk of the friction roller. Due to the variation of these contact points, the input disk and output disk exert a component force which varies the gyration angle of the friction roller, and as a result, the gyration angle of the friction roller varies.
Servo pistons are fixed to the trunnions. The trunnions displace according to the differential pressure of oil chambers on both sides of the servo piston, and the variation of the gyration angle of the friction roller, i.e. the variation of the speed ratio, is faster the larger the differential pressure. A speed ratio control valve selectively connects the two oil chambers on either side of the servo pistons to an oil pressure source and a drain according to the displacement of a spool. The spool is joined to a step motor, and the friction roller is controlled to a gyration angle corresponding to the target speed ratio by inputting a signal corresponding to the target speed ratio to the step motor.
If the spool of the speed ratio control valve sticks in a specific position, or faulty operation occurs due to disturbance of the step motor, the speed ratio may vary sharply. In particular, if the spool sticks in a downshift position, a sharp downshift unrelated to the driver""s intention is produced.
Similarly, when the spool sticks in an upshift position, a sharp upshift may occur, but as a downshift causes engine braking, the uncomfortable feeling given to the driver is greater for downshift than for upshift.
The differential pressure can be prevented from becoming excessive by controlling the stroke range of the spool. However if the stroke range of the spool is limited, it is possible that the required differential pressure may not be produced when the oil pressure of an oil pressure source drops.
It is therefore an object of this invention to prevent an excessive downshift pressure from acting on a piston without restricting the stroke range of the spool of the speed ratio control valve.
In order to achieve the above object, this invention provides a controller for a toroidal continuously variable transmission of a vehicle. The transmission comprises an input disk which rotates about a rotation shaft, an output disk which rotates about the rotation shaft, a power roller gripped by the input disk and the output disk and transmitting a torque between the input disk and output disk, and a trunnion which drives the power roller in a direction perpendicular to the rotation shaft according to a differential pressure of a first oil chamber and a second oil chamber. The transmission causes a downshift when the second oil chamber is at a higher pressure than the first oil chamber, and causing an upshift when the first oil chamber is at a higher pressure than the second oil chamber. The first oil chamber is connected to a first passage and the second oil chamber is connected to a second oil passage. The controller comprises a speed ratio control valve which controls a direction and a flowrate of oil in the first oil passage and the second oil passage, and a pressure control valve which limits a maximum differential pressure of the first oil chamber and the second oil chamber when the transmission causes a downshift smaller than a maximum differential pressure of the first oil chamber and the second oil chamber when the transmission causes an upshift.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.